According to well-known prior art IC (integrated circuit) packaging methodologies, semiconductor dice are singulated and mounted using epoxy or other conventional means onto respective die pads (attach paddles) of a leadframe strip. Traditional QFP (Quad Flat Pack) packages incorporate inner leads which function as lands for wire bonding the semiconductor die bond pads. These inner leads typically require mold locking features to ensure proper positioning of the leadframe strip during subsequent molding to encapsulate the package. The inner leads terminate in outer leads that are bent down to contact a mother board, thereby limiting the packaging density of such prior art devices.
In order to overcome these and other disadvantages of the prior art, the Applicants previously developed a Leadless Plastic Chip Carrier (LPCC). According to Applicants' LPCC methodology, a leadframe strip is provided for supporting up to several hundred devices. Singulated IC dice are placed on the strip die attach pads using conventional die mount and epoxy techniques. After curing of the epoxy, the dice are gold wire bonded to peripheral internal leads. The leadframe strip is then molded in plastic or resin using a modified mold wherein the bottom cavity is a flat plate. In the resulting molded package, the die pad and leadframe inner leads are exposed. By exposing the bottom of the die attach pad, mold delamination at the bottom of the die attach pad (paddle) is eliminated, thereby increasing the moisture sensitivity performance. Also, thermal performance of the IC package is improved by providing a direct thermal path from the exposed die attach pad to the motherboard. By exposing the leadframe inner leads, the requirement for mold locking features of outer leads is eliminated and no outer leads are necessary, thereby increasing device density and reducing package thickness over prior art methodologies. The exposed inner leadframe leads function as solder pads for motherboard assembly such that less gold wire bonding is required as compared to prior art methodologies, thereby improving electrical performance in terms of board level parasitics and enhancing package design flexibility over prior art packages (i.e. custom trim tools and form tools are not required). These and several other advantages of Applicants' own prior art LPCC process are discussed in Applicants' U.S. Pat. No. 6,229,200, issued May 8, 2001, the contents of which are incorporated herein by reference.
In use, the exposed die attach pad and contact pads of Applicant's previously developed LPCC are soldered to the motherboard. To facilitate soldering of the exposed die attach pad and the contact pads, solder paste is printed on the exposed surface of the die attach pad and on the contact pads. The solder paste is reflowed during connection of the package to the motherboard to thereby form a solder joint between the package and the motherboard. During reflow, surface tension of the solder paste on the large die attach pad causes reduced area of coverage of solder paste on the die attach pad and increased height of the solder paste between the die pad and the motherboard. This results in lifting of the package, weakening of the solder attachment between the die attach pad and the motherboard and in extreme cases, causes opening of the input/outputs or disconnection of the contact pads from the motherboard due to increased gap height between the package and the motherboard. Reflowing of solder printed on the surface of the die attach pad and the contact pads results in a solder bump height difference between the solder bump on the large die attach pad and the solder bumps on the smaller contact pads due to surface tension of the solder. Clearly this height difference is undesirable.
Further IC package improvements are desirable and are driven by industry demands for increased reliability, improved thermal and electrical performance and decreased size and cost of manufacture.